Degrade gracefully during conflict or removal

See merge request dev/civicrm-asset-plugin!6

Before this patch, when uninstalling the plugin on composer v1, it would raise an error like:

```
Writing lock file
Generating autoload files
  - CiviCRM asset map
PHP Fatal error:  Uncaught Error: Class 'Civi\AssetPlugin\BasicAssetRule' not found in /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php:187
Stack trace:
0 /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php(215): Civi\AssetPlugin\Publisher->createAssetRule(Object(Composer\Package\CompletePackage))
1 /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php(98): Civi\AssetPlugin\Publisher->createAllAssetRules()
2 phar:///home/me/bknix/bin/composer/src/Composer/Plugin/PluginManager.php(196) : eval()'d code(125): Civi\AssetPlugin\Publisher->createAssetMap()
3 [internal function]: Civi\AssetPlugin\AssetPlugin_composer_tmp2->onAutoloadDump(Object(Composer\Script\Event))
4 phar:///home/me/bknix/bin/composer/src/Composer/EventDispatcher/EventDispatcher.php(164): call_user_func(Array, Object(Composer\Script\Event))
5 phar:///home/me/bknix/bin/composer/src/Composer/EventDispatcher/EventDispatcher.php(96): Composer\EventDispatcher\Event in /home/me/src/sandbox/vendor/civicrm/civicrm-asset-plugin/src/Publisher.php on line 187
```

Ensure that asset-plugin sync's assets after compile-plugin has a chance to build

See merge request dev/civicrm-asset-plugin!5

Memory utilization of Drupal's default template is not really the fault of this plugin.

Update to support composer-plugin-api v2

See merge request dev/civicrm-asset-plugin!4

Address Array and string offset access syntax with curly braces is deprecated

See merge request dev/civicrm-asset-plugin!3

dev/core#1643 - Prevent broken paths on windows during civicrm-setup

See merge request dev/civicrm-asset-plugin!2

A couple unlikely edge-cases:

* A dependency is installed in a relative path with one letter (eg `myproject/c`)
* A dependency is installed in a relative path with a colon (eg `myproject/::stuff::`)

Added mitigation for the first and comment to help identify the second.

The upstream interpretation of `$civicrm_paths['civicrm.packages']['url']`
currently requires a trailing slash...  but `autoload_civicrm_asset.php` may
not provide trailing slashes depending on the happenstance of
defaults+customizations.

This patch updates the generated values for `$civicrm_paths` and
`$civicrm_asset_map` so that they consistently use (or do not use) trailing
slashes.

(drupal8-clean) The JS/CSS assets are sync'd to web dir, but the $civicrm_paths should still autoload PHP from the src folder.

This improves support for drupal8-clean:

1. The content of `$civicrm_paths['civicrm.packages']['path']` should be the
   *source*, because that has all files (including PHP and JS/CSS)
2. Fix generation of quasi-relative paths

There is no perfect file-mode:

* `copy` is very compatible, but it's annoying for development. (You have to republish
  after every change.)
* `symdir` is very convenient for developyment, but it's not good for production.
  (You rarely have to manually publish -- but it exposes extra files to the web.)
* `symlink` is middle-ground. (It doesn't expose extra files, and you may have to
   republish more times, but not as much.)
* Both `symlink` and `symdir` depend on the platform, filesystem, and HTTPD options.

This patch allows the following recommendation:

* Set the default to `copy`, and use that default for production/staging/CI.
* On local dev machines, set CIVICRM_COMPOSER_ASSET=symdir so that you never
  have to tweak the configuraiton for a particular build.

Ex: `composer-extra-files` downloads CSS assets in
`vendor/civicrm/civicrm-core`; then `civicrm-asset-plugin` copies them to
`web/libraries/civicrm/core`.

USE_TEST_PROJECT - Document it. Remove KEEP_TEST_PROJECT, since the use-case is handled equally well by USE_TEST_PROJECT.